Low power on startup and KSOD Vista X64

Can any one help with my start up problem?
Using Vista X64 Home Premium

For my desktop pc I have a power and boot up problem that does like this……….

Turn on main front power button
BIOS screens run through all OK
Windows startup screen runs OK (with the flashing green/yellow bar)
Then what normally happens is a pause of one or two seconds before the Windows icon appears and I can login.

What has been occurring recently is at this pause, the screen stays black and the computer goes into what can only be described as a low power mode. I sense this as the fan speed reduces, sometimes uneven and the screen stays black. Nothing else will happen.

BUT if I turn the pc off at the main power button at the rear and switch on again it will boot up, all through the processes with no problems. However if I do this at the stage above I have effectively forced a shut down and Windows enter via the recovery, but this still works fine. Windows etc works then with no problems at all.

Also if after shutting the PC down correctly I turn off the rear power button and turn it on before start up, all starts OK.

No devices are connected and powered on at start up other than Netgear router. I have disabled the devices (Control panel, Device manager, Properties) to “wake” the computer (mouse and keyboard) and also the Network Adapter, all of which were causing problems in waking the computer. This was a while ago and has been working fine for many months.

I have run diagnostics on memory, hard drives and fans and all OK. On restart boots up all OK to Windows login.

Any good ideas would be appreciated. Could it be a software problem (drivers?) or a hardware problem (power supply?)

You could try booting a linux live cd and see if that os detects any hardware issues on boot. If you used fedora live cd, one thing is that disk SMART reporting is on by default and will alert you to any SMART errors reported by the drive. I mention this because your description sounds like a hardware issue to me.

When the computer is booted normally, try running chkdsk if that wasn't one of the diagnostics you already ran.

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